In research laboratories and in different areas of industry such as chemicals, food, biotechnology or pharmaceuticals, measurement electrodes or measuring probes are used to determine the pH-value in a measurement medium. The pH-value of a measurement medium is indicative of the condition, taste or quality of the measurement medium. In chemical processes, the pH-value is also used to control the manufacturing processes.
The pH measurement is carried out according to the principle of a potentiometric measurement. In a potentiometric measurement, a measurement electrode (ISFET) and a reference are immersed in the measurement medium. Through the reference electrode, a defined potential is set up relative to the measurement medium. By measuring, the difference between the respective potentials of the reference electrode and the measurement electrode, the pH-value of the measurement solution can be determined.
Reference electrodes include for example a housing containing a reference electrolyte which, by way of an interface, is in contact with a measurement medium. The interface can be for example an open passage, or it can be configured as a porous diaphragm. The interface should on the one hand have a minimal electrical resistance and should on the other hand impede the mixing of the reference- or bridge electrolyte with the measurement medium. Different measures aim to accomplish these objectives.
The diaphragm of the measuring probe which is in direct contact with the medium on which the measurement is performed is a very sensitive part of the measuring probe. A known problem is the so-called bio-fouling, i.e. foreign substances settling and accumulating on the diaphragm surface and interfering with the measurement. The acidity, fat content, volume and pressure of the measurement medium are factors that can have a negative effect on the operation and on the useful life of the diaphragm.
It is an essential quality of reference electrodes that they maintain, as much as possible, a constant reference potential. This requires that the diaphragm has a constant electrical resistance that should be as small as possible. It is therefore advantageous to check the condition of the diaphragm regularly during operation in order to obtain reliable measurements of the ion concentration.
In pH glass electrodes, diagnostic circuits are used for the purpose of monitoring the condition of the diaphragm and providing status messages, so that the user can terminate the measurement and exchange the sensor at the right time.
Glass electrodes have the disadvantage that the existing diagnostic circuits require additional components. They generate heat which compromises the measurement results and makes it necessary to take corrective measures.
Glass electrodes have the further disadvantage that they are very fragile. In the case of a breakage, the glass fragments would become mixed up in the process material and would consequently pose a danger to the consumer. The use of glass electrodes is therefore normally avoided in a number of areas, for example in biopharmaceuticals and in the food- and beverage industry. Due to these problems, the use of glass electrodes is even legally banned in some areas of industry.
Ion-sensitive field effect transistors (ISFET) are better suited than glass electrodes for these applications, because they are mechanically very stable and nearly unbreakable in comparison to glass electrodes. These properties favor the use of ISFET sensors in areas such as for example the food industry and the pharmaceutical industry, where it is particularly important to protect the process material.
The determination of the pH-value of a measurement medium with a measuring probe that includes an ISFET is based on measuring the behavior of the ISFET. The latter is arranged in the measuring probe so that the gate, which is covered with an ion-sensitive layer, can be brought into contact with the measurement medium. The measuring probe further includes a reference electrode which is likewise immersed in the medium and to which a biasing voltage is applied that determines the electrical potential of the measurement medium and thus the operating point of the ISFET. According to Nernst's equation, a surface potential which depends on the ion concentration in the measurement medium establishes itself in the contact area between the measurement medium and the ion-sensitive layer at the gate of the ISFET. This potential adds itself to the biasing voltage that is applied constantly to the reference electrode and thus affects the depletion zone between source and drain.
Under the conventional method of measuring the pH-value of a measurement medium by means of an ISFET the drain current and the drain potential are held constant, and the source potential is measured. As an alternative, the bias voltage at the reference electrode can be varied and the drain current can thereby be held constant. In this case, the regulated bias voltage serves as the measure for the ion concentration that is present.
Measuring probes with an ISFET have a diaphragm which allows the reference electrode to have access to the measurement medium. For the measurement of the ion concentration to deliver valid results, it is necessary to ensure that a change in the condition of the diaphragm due to detrimental influence factors such as acid, grease and pressure does not affect the measurement.
The condition of the diaphragm can be determined by measuring its electrical resistance. In state-of-the-art diagnostic procedures for ISFET measuring probes the measuring probe has to be taken out of the measurement medium in order to examine the condition of the diaphragm.
The object of the invention is to propose an improved ISFET measuring probe, in particular a pH-measuring probe, as well as a measurement circuit and a method whereby the condition of this measuring probe can be determined.
In particular, the invention aims to provide a diagnostic circuit which allows an in-situ diagnosis, i.e. a diagnosis which does not require the measuring probe to be removed from the measurement medium and which examines the condition of the reference electrode during operation. For reliable measurement results, the diagnostic circuit should only have a small number of components and should only consume a minimal amount of electrical current.